I listen to a good bit of Chinese and Japanese pop. One thing I've noticed is that both are, on occasion, wont to sing in English. As far as I've noticed, Chinese singers are much better at singing in English than Japanese singers. If I were a singer, I would think that singing poorly in a foreign language would be embarassing.
Chinese singers, the ones I've heard, aren't the best English singers, but they're actually very passable. Sometimes it's Chinglish (a mixture of Chinese and English), and sometimes it's a complete song in English. Far from having accent problems, I think they just wouldn't be my first choice for singing an English song.
Japanese singers, on the other hand, sing an English word poorly and with pride. These songs are mostly Japlish. Actually, they'd mostly be Japanese with a word of English tossed in here and there. One hilarious offender is a song from an anime called Hokuto no Ken, You wa Shock.
Come to think of it, I seem to recall reading something about how European singers are taught to sing with an American accent. From the few Korean songs I've listened to, their English seems fine. I even recall reading how some Koreans undergo surgery for their tongue to overcome physical limitations in their pronunciation. The one Vietnamese singer I listen to also has great English pronunciation. Spanish singers sing English songs all the time, and songs like "She Bangs" aside, they sound fine.
So what's up with the Japanese?